So far proposed heat-sensitive recording systems include a wide variety of embodiments. For example, heat-sensitive recording materials using a color former and a color developer are disclosed in U.S. Pat. Nos. 4,255,491, and JP-B-43-4160 and JP-B-45-14039 (the term "JP-B" as used herein means an "examined published Japanese patent application"). With the recent tendencies to rapids reduced energy heat-sensitive recording systems, extensive studies have been directed to an increase of sensitivity of heat-sensitive recording materials. In this connection, many attempts have been made to increase sensitivity by utilizing various additives or sensitizers, and the inventors of the present invention have filed patent applications on several such compounds [e.g., JP-A-58-57989 (corresponding to U.S. Pat. No. 4,480,052), JP-A-58-87044 (corresponding to U.S. Pat. No. 4,471,074), and JP-A-61-123581 (the term "JP-A" as used herein means an "unexamined published Japanese patent application")].
On the other hand, there is a tendency that the temperature at which color formation of a heat-sensitive recording material initiates decreases as the sensitivity of a recording material increases. This tendency not only leads to undesired color formation on white background after black image formation with facsimiles, etc. but also gives rise to a problem relating to preservability at high temperatures. It has been therefore desirable to develop a heat-sensitive recording material exhibiting high sensitivity while retaining satisfactory preservation stability.